The present invention relates, in general, to spray coating apparatus of the electrostatic type, and more particularly to an adapter for use in converting conventional liquid spray guns to electrostatic use wherein the spray particles are subjected to an electrostatic field which induces a charge on the particles.
Induction charging of liquid spray particles is a technique that is known in the art, and is discussed in copending application Ser. No. 456,944 of James E. Sickles, filed Apr. 1, 1974 and assigned to the assignee of the present application now abandoned. As stated in that application, the principle of providing an electrostatic field for charging atomized liquid particles and utilizing the charged particles for coating an article has been in general use for many years. A great variety of spray gun, configurations have been devised for such systems, some of which work reasonably well and some of which are less than satisfactory. Virtually all of these prior electrostatic devices have in common a spray gun to which is mounted a high voltage electrode disposed adjacent the spray discharge point and carrying an electrical potential in the neighborhood of from 50 to 85 kilovolts, and in certain instances as high as 150 kilovolts. The voltage on this electrode creates a corona discharge condition and the resulting electric field creates a region rich in ions through which the spray particles must pass. Some of these ions become attached to the spray droplets, producing electric charges on the particles which may then be directed toward a workpiece which is electrically grounded and which therefore attracts the particles.
Use of the corona discharge type of electrostatic spray coating apparatus presents numerous difficulties, principally as a result of the very high voltages that must be utilized to produce effective operation. First of all, the requisite charging voltages are conventionally obtained through the use of standard electronic high voltage power supplies which are relatively large, heavy, and expensive. Because of the high voltages involved, the cable interconnecting the power supply and the spray gun charging electrode must necessarily be heavily insulated and thus is bulky, relatively inflexible, and, again, very expensive. The size and weight of the power supply and its cable substantially restrict the usefulness of the conventional corona effect spray gun because of the difficulties encountered in handling and moving it, and the high cost limits its availability. Furthermore, the use of such high voltages is hazardous, not only because of the possibility of creating electrical arcs when the gun is moved too near a grounded object, but because of the possible danger to the operator should he inadvertently touch the high voltage electrode. Finally, the high voltages used in such systems create an excessive current flow in the form of ions which may travel to nearby objects, resulting in undesired charge buildup on any objects that are not adequately grounded. The hazard of sparking and consequent possible fire exists when the operator or some other grounded object is brought close to such a charged object. Further, the migration of such charges can cause an undesired buildup of the charged spray particles on objects other than the workpiece. Such a buildup can occur on the grounded spray gun itself and can result in clogging of the gun and premature shutdown for cleaning.
As set forth in the above-mentioned copending application, it has been found that effective electrostatic spray coating can be accomplished through the use of induction charging apparatus which eliminates the need for the very high voltages used in the corona discharge type of electrostatic charging. Induction charging of liquid particles in spray discharge devices is accomplished by surrounding the discharged spray with a static electric field which has an average potential gradient in the range of about 10 to 20 kilovolts per inch, with the liquid being held at ground potential. The spacing between the liquid and the source of potential is sufficient to prevent an electrical discharge so that a capacitive-effect is produced. The static field so provided induces on liquid particles produced within the field an electrical charge having a polarity which is opposite to that of the applied voltage, with the particles carrying quantities of the charge. The resulting charged particles may then be directed, for example, at an electrically grounded workpiece, striking it to provide a coating of the liquid on the workpiece. Such induction charging techniques have been found to be particularly useful in spray systems utilizing electrically conductive liquids such as water base paints, since the liquid can be electrically grounded. This is a considerable improvement over corona discharge and other high voltage spray devices which utilize a high voltage needle electrode in contact with the liquid. In such devices, the liquid is at the same high voltage as the electrode, requiring that the liquid be electrically isolated to prevent excessive current flow and to insure the safety of the operator. Such isolation not only requires bulky and expensive electrical insulation, but produces a system which is inconvenient to use. The lower voltages used and the grounded of the liquid supply in an induction type of system eliminates the problems inherent in high-voltage isolated systems, thus reducing the danger of operator injury, providing increased convenience and flexibility in use, reducing the danger of arcing if the spray gun is moved too close to a grounded object, and reducing the intensity of the arc, and the danger of fire, if arcing should occur.
To enable conventional non-electrostatic spray guns, as well as spray guns of the very high voltage, corona discharge type, to be converted to induction charging devices, the aforementioned copending application described an adapter which may be secured to a spray nozzle to surround the discharge ports of the spray gun and to produce a charging zone through which pass liquid particles exiting from the nozzle discharge ports. The adapter is located exteriorly of the conventional air and liquid discharge ports and is spaced radially outwardly therefrom. In the copending application, the inductive charging means was illustrated as including a cylindrical dielectric tube having a thin conductive film such as a metallic foil adhered to the interior surface thereof, the tube circumferentially surrounding the discharge ports to define a charging zone. Means were provided in that device for applying between the conductive film and the electrically grounded liquid a d.c. voltage of less than about 20 kilovolts to produce a preferred average potential gradient within the charging zone of between about 5 to about 20 kilovolts per inch.
Although the inductive charging device of application Ser. No. 456,944 operates satisfactorily, it has been found that the cylindrical shape in some operational modes interferes with the discharge pattern of the spray, with the result that in such situations the inductive charging device can become coated with the spray material, thereby reducing operating efficiency. Further, it has been found that the collection of charged particles on the outer surface of the inductive charging tube over a prolonged period of operation could produce surface leakage paths between the high voltage electrode and ground, thereby reducing the voltage available for operation and making it necessary to shut down the unit for cleaning. Finally, because the induction charging device of the above-mentioned copending application utilized a moderately high voltage, some slight danger of injury to the operator or of arcing remained, even though such dangers were substantially reduced from the problems produced by prior very high voltage systems.